Ketamine produces effects similar to phencyclidine (PCP) and dextromethorphan (DXM). Unlike the other well-known dissociatives PCP and DXM, ketamine is very short-acting, its hallucinatory effects lasting sixty minutes when insufflated or injected and up to two hours when ingested, the total experience lasting no more than a couple of hours.[35] Like other dissociative anaesthetics, hallucinations caused by ketamine are fundamentally different from those caused by classical hallucinogens. At low doses, hallucinations are only seen when one is in a dark room with one's eyes closed, while at medium to high doses the effects are far more intense and obvious.[36]

Ketamine produces a dissociative state, characterised by a sense of detachment from one's physical body and the external world which is known as depersonalization and derealization.[37] At sufficiently high doses (75-125 mg IM), users may experience what is called the "K-hole", a state of dissociation whose effects are thought to mimic the phenomenology of schizophrenia.[38] Users may experience worlds or dimensions that are ineffable, all the while being completely unaware of their individual identities or the external world. Impressions of the K-hole often include profound distortions in or complete loss of bodily awareness, sensations of floating or falling, euphoria, and total loss of time perception. John C. Lilly,[39]Marcia Moore[40] and D. M. Turner[41] (amongst others) have written extensively about their own entheogenic use of, and psychonautic experiences with, ketamine. (Both Moore[42] and Turner[43] died prematurely in a way that has been linked to their ketamine use.)

Users may feel as though their perceptions are located so deep inside the mind that the real world seems distant. Some users may not remember this part of the experience after regaining consciousness, in the same way that a person may forget a dream. Owing to the role of the NMDA receptor in long-term potentiation, this may be due to disturbances in memory formation. The "re-integration" process is slow, and the user gradually becomes aware of surroundings. At first, users may not remember their own names, or even know that they are human, or what that means. Movement is extremely difficult, and a user may not be aware that he or she has a body at all.

Ketamine is considered as an anaesthetic with a good safety profile, based on extensive clinical experience. The major drawback, limiting clinical use, is the occurrence of emergence reactions in patients awakening from ketamine anaesthesia. These reactions include hallucinations, vivid dreams, floating sensations and delirium. However, preclinical data on the effects of repeated ketamine administration may be of greater importance for recreational use which, contrary to clinical practice, may present cases of long-term use.

A total of 12 deaths in which ketamine was identified, have been noted between 1987-2000 including seven from the USA. Only three reported fatal cases involving ketamine alone were identified. Two reports concern mixed drugs fatalities. In one case,ketamine had only a minor role. For the remaining six cases, insufficient details were available to be evaluated properly. In the three cases involving only ketamine, the routes of administration were intramuscular or intravenous and the cause of death was mainly due to overdose (multiple intramuscular doses or accidental intravenous overdose), in line with preclinical findings. In the other cases involving ketamine mixed with other drugs, the observed lower ketamine concentrations indicates that drug interaction may have contributed to these deaths. Substances with CNS/respiratory depressant effects, like ethanol, opioids, barbiturates, and benzodiazepines or drugs with cardiostimulant effects, like cocaine or amphetamines, may increase ketamine
acute toxicity.

Regarding non-fatal intoxications, potential dangerous interactions may also arise when different drugs are combined. Ketamine has sympathomimetic properties. Inhibition of central catecholamine re-uptake and increased levels of circulating catecholamines are believed to cause the cardiovascular stimulant effects. Serious side effects such as hypertension and pulmonary oedema have been reported, but such adverse effects appear to be rare and may be related to the combination of ketamine with other drugs, such as amphetamines and its analogues, ephedrine and cocaine.

...

The most vulnerable groups are those who take ketamine under the illusion they are taking MDMA or some other stimulant drug. The volume of seizures of ketamine in tablet form with ecstasy-type logos reflects the scope for this scenario and the need for better information about drug contents and harm reduction. Ketamine does not react with commonly used field tests (e.g., Marquis reagent) although other drugs present in the tablet may produce a positive reaction.

...

In the EU since 1996, there have been four deaths reported to the EMCDDA in which ketamine was found by laboratory analysis, of which two occurred in 1996 in Ireland. In neither of the Irish cases, ketamine was considered to be the main cause of death. One death of a 19 year-old-male has been reported in France where ketamine, LSD and ecstasy were implicated. The fourth death, also reported from France, was a polydrug user.

There has been a notable lack of reporting about hospital emergencies in Europe. A recent report in France presents some data on 17 cases of intoxication associated with ketamine.

An important factor of health risk is the lack of reliable indications of dose accompanying sales of ketamine at street level. In the absence of advice, first time users of ketamine will tend to follow similar consumption patterns as those previously adopted for other drugs. This uninformed use of ketamine increases the risk of both physical and psychological problems. The existence of tolerance may increase a tendency to move from snorting to injecting ketamine, with the risks associated with injection. [mine markeringer]

The LD50, or Median Lethal Dose (50% of users will die at this dosage level) for ketamine is 400mg/kg. Remember, an IM K-hole does is 60-125 mg for an average user. This means that a 200 pound, or 90 kilogram user would need to administer 36,000mg or 36 grams. That’s about 360 normal doses to OD. Overdose is not an issue with ketamine. What is common is for acquaintances unfamiliar with the effects of ketamine finding a user unresponsive and assuming the user is in danger. Again the importance of a safe environment and a clear-headed sitter are to be stressed. [kilde]